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Diaz Montano, John

Abstract

Onion thrips, Thrips tabaci Lindeman, a worldwide pest of onion, Allium cepa L., can reduce onion yield by &gt;50% and transmit Iris yellow spot virus (IYSV), confirmed in 2006 in New York. T. tabaci are difficult to control with conventional insecticides and have developed resistance to some insecticides in New York. For these reasons, it is necessary to develop a research program aimed at finding alternative management strategies for T. tabaci and IYSV. In field studies over a 2-year period, 49 onion cultivars were screened and 11 had very little leaf damage and were considered resistant to T. tabaci. Choice and no-choic e tests were performed to characterize resistance of different cultivars to T. tabaci. Resistant cultiv ars showed an intermediate to high antibiotic effect to T. tabaci and a very strong antixenotic effect. A study on behavioralr esponses of walking T. tabaci adults using a Y-tube olfactometer suggested that there is not an oriented movement towards onion plant odors. To detect the presence of IYSV transmitted by T. tabaci, in a laboratory experiment, onion cultivars were infested with T. tabaci larvae from an IYSV-infected onion field. In a complementary experiment, plants were moved to an infected IYSV field. All the cultivars became infected with IYSV. The infection varied from 3 to 25% and 37 to 70% in the laboratory and field experiments, respectively. It was observed that cultivars that were identified as resistant to T. tabaci were not necessarily resistant free of the virus and vice versa. Visual assessment indicated that all resistant cultivars had yellow-green colored foliage, whereas susceptible ones had blue-green colored foliage. The reflectance spectrum was measured on leaves using a spectrometer in order to determine if light reflectance was associated with resistance to T. tabaci. Susceptible cultivars had the highest values of leaf reflectance in the first (275-375 nm) and second (310-410 nm) theoretical photopigmen t-system of T. tabac i and these values were significantly different from most resistant cultivars. These results indicated a strong response of T. tabaci to onion cultivars with higher reflectance in the UV range (270-400).>